A wireless power transmission system has a radio-frequency (RF) transmitter and an RF receiver. The RF transmitter is coupled to a source of electrical power (e.g., a mains power line), and it converts an electrical current into an oscillating electromagnetic field using an inductor, coil, antenna, metal plate, or other coupling device. Another coupling device at the RF receiver captures a portion of the radiated electromagnetic field (the two coils in proximity to each other form an electrical transformer), and the RF receiver then converts the received electromagnetic field back into an electrical current.
In many implementations, the RF transmitter may be disposed within a charging pad or station, and the RF receiver may be coupled to a computing device (e.g., a desktop, laptop, tablet, smart phone, etc.) or a battery. The energy transferred between the RF transmitter and the RF receiver may then be used to operate the computing device and/or to charge the battery. Hence, transfer efficiency (the amount of energy that is received relative to the amount of energy that is transmitted) is a critical parameter in wireless power transfer design.
As the inventors hereof have recognized, coils in both the RF transmitter and the RF receiver are controlled using a number of switches or transistors, and increasing the power transfer efficiency often requires precise control of those transistors. However, the high-frequency clocks that would typically be necessary to provide such control would significantly increase cost and complexity. To address these, and other concerns, the inventors have developed systems and methods for wireless power transfer with fractional timing resolution.